Capacitive charge pump, Bicmos circuit for low supply voltage

Abstract: A BiCMOS capacitive charge pump circuit for low supply voltage has a bipolar part, functionally reproducing a basic charge pump circuit and a CMOS part that comprises MOS transistors (M1, M2) functionally connected in parallel with the driving switch toward ground potential (T3) of the charge transfer capacitance (C1) and in parallel with the output diode (T7) for substantially nullifying voltage drops on the respective bipolar components (T3, T7). A special driving circuit (T8, R2, I2), powered at the boosted output voltage (V OUT ) responds to the rise of the voltage on the output node above a minimum level, as ensured by the bipolar part of the charge pump circuit, to drive said MOS transistors (M1, M2), thus allowing the output voltage to reach a level that is substantially double the supply voltage (Vs), also when the latter is exceptionally low for reliably ensuring switching of the CMOS part of the circuit.

Description: The invention relates to a capacitive charge pump circuit employing complementary bipolar transistors and complementary field effect transistors (BiCMOS) capable of ensuring a substantial multiplication of the supply voltage also when it is exceptionally low. Capacitive charge pump circuits are widely used in electronic systems for generating voltages higher than the supply voltage. The capacitive charge pump circuit is basically a voltage duplicating circuit that may be used either singularly as voltage duplicator as well as a stage or cell to be connected in cascade to other similar cells for realizing multi-stage voltage multipliers. Basically, as shown in Figure 1, a capacitive charge pump circuit is composed of a charge transfer (pumping) capacitor C1, connected to the intermediate node of a pair of diodes D1 and D2, connected in series between a supply (input) node and an ouput node, to which a storage capacitor C2 is connected. A pair of switches SW1 and SW2, driven in phase opposition to each other and at a certain frequency, switch the charge transfer capacitor, alternatively to ground potential (charge phase) and to the supply node (charge transfer phase). By neglecting the voltage drop on the diodes D1 and D2, the circuit is theoretically capable of providing an output voltage V OUT double the supply voltage V S . Such an ideal result assumes that, beside neglecting the voltage drop on the diodes D1 and D2, also the voltage drop on the driving switches SW1 and SW2 when closed be negligeable. More precisely, the maximum open circuit output voltage that can be obtained is given by: V OUT = 2V S - 2V diode - V SW1 - V SW2 Only with a relatively high supply voltage V S , the above noted voltage drops may be reasonably neglected. Certainly this condition hardly exists in low voltage, battery powered systems. In portable apparatuses, as for example in electrical wrist watches, acoustic protesys, sensors and similar the electronic circuits, displays and actuators are often powered with small batteries at a nominal voltage that may often be comprised between 1.2V and 1.4V. With almost exausted batteries, the supply voltage may even drop toward 1.0V. In these conditions, it is evident that the above noted voltage drops on the circuit components that form a capacitive charge pump circuit assume great importance up to make critical the operation of the circuit itself. If the circuit is realized with a "bipolar technology", that is by employing bipolar transistors as shown in Figure 2, the switches may be implemented in the form of a complementary pair of bipolar transistors T1 and T2 respectively, driven in phase opposition. In view of the fact that their offset voltage is typically of about 0.7V (VBE = 0.7V), they can ensure switching also with a supply voltage of about 1V. On the other hand, the voltage drop on the diodes D1 and D2 and on the driving switches themselves (T1 and T2) do not allow, with a supply voltage in the order of 1-1.5V, a substantial duplication of the output voltage V OUT and the efficiency of the circuit drops drastically. In order to obviate to this typical limitation of a charge pump circuit made with bipolar components, it is known to realize the circuit with field effect devices, for example with MOSFETs, that is by realizing the integrated circuit with a CMOS technology, as shown in Figure 3. The diodes D1 and D2 of the functional circuit of Figures 1 and 2 are substituted by MOS transistors M3 and M4 which virtually form a "syncronous rectifier", being the MOS transistor virtually free of offset. By realizing the driving switches with a CMOS pair of transistors M1 and M2, the circuit is theoretically capable of producing an output voltage V OUT double the supply voltage V S . This alternative solution that is certainly advantageous in terms of "yield", has the drawback represented by the fact that with an exceptionally supply voltage, the circuit may cease to work by failing to switch. In fact, considering that the threshold value of the complementary pair of transistors M1 and M2 is strongly dependent on the temperature of operation, a supply voltage of at least 1.2-1.3V may be required for ensuring the switching of the driving switches M1 and M2. In many apparatuses, it is important to ensure a correct operation also under precarious charge conditions of the battery (approaching exaustion), that is with a supply voltage that may drop in the vicinity of about 1.1-1.0V and clearly, in these applications a CMOS circuit as the one depicted in Figure 3, cannot be employed because it would make critical the operation of the circuit at exceptionally low supply voltages. It is the main object of the invention to provide a capacitive charge pump circuit particularly suited for low supply voltage applications, capable of ensuring a correct operation also with a supply voltage or input voltage well below the limit of correct operability of a CMOS circuit, though being capable of producing a substantial doubling of the supply voltage also under such critical supply conditions. This objective is fully met by the circuit of the invention that can be realized with a mixed fabrication technology (BiCMOS), and which employs bipolar transistors and field effect transistors. Basically the circuit of the invention has a "bipolar portion" intrinsically capable of ensuring switching also with a supply (or input) voltage close to or equal to 1.0 Volt, and a CMOS portion, capable of practically nullifying or markedly reduce the voltage drops through the bipolar components of the circuit and therefore to permit a rise of the boosted output voltage close to a theoretically doubling of the supply (input) voltage. In practice, the bipolar part of the circuit ensures operability of the circuit when it is first switched on, by producing a rise of the boosted output voltage, whenever the supply (or input) voltage is below the minimum value for a correct functioning of the CMOS part of the circuit. The substantial rise of the output voltage over the actual supply voltage that is eventually guaranteed by the bipolar part of the circuit, is sufficient to permit switching in the CMOS part of the circuit, which by intervening to essentially eliminate voltage drops, permits to the circuit as a whole to reach a steady state working condition whereby it produces a substantial doubling of the supply (or input) voltage. Essentially the bipolar part of the charge pump circuit of the invention is different from a typical charge pump bipolar circuit because the first diode (D1 of Figures 1 and 2) is substituted by a bipolar transistor driven by a bipolar stage controlled by the switching (clock) signal. The CMOS part of the circuit of the invention essentially consists of a first field effect transistor connected in parallel and driven in phase with the bipolar transistor that constitutes the switch toward ground of the charge transfer capacitor and by a second field effect transistor, functionally connected in parallel with the charge diode of the output capacitor, driven in phase with the charge transfer from the charge transfer capacitor to the output storing capacitor. The CMOS pair of transistors is driven by the switching (clock) signal through a level shifting bipolar stage followed by an inverting stage, both powered with the voltage present on the output node of the charge pump circuit. The second inverting stage may be realized by a CMOS inverter, though it may also be realized by a second bipolar stage. As soon as the voltage present on the output node of the circuit reaches and becomes higher than the minimum voltage for overcoming the turn-on threshold of the MOS transistor or of the MOS transistors that form the second inverting stage, or, in case that also the second inverting stage is bipolar, the turn-on threshold of the CMOS pair of transistors, functionally connected in parallel with the respective bipolar elements, these CMOS transistors begin to switch, practically eliminating the relative voltage drops. The operation of the circuit under critical conditions of the input voltage, for example when switching on the circuit and/or in presence of a battery close to exaustion, is ensured by the bipolar part of the capacitive charge pump circuit, which is capable of rising the voltage on the output mode above the supply or input voltage by an amount sufficient to drive the CMOS part of the circuit which thereafter ensure a substantial duplication of the supply voltage (or of the input voltage in case of a multi-stage voltage multiplier). In case of a multi-stage voltage multiplier circuit, the basic BiCMOS circuit (cell or module) of the invention may be effectively employed as a first stage. The other stages of the voltage multiplier may be realized entirely with a CMOS technology (for example as shown in Figure 3). The different aspects and advantages of the circuit of the invention will become more evident through the following description of an important embodiment and by referring to the annexed drawings, wherein: Figure 1 is a basic diagram of a capacitive charge pump circuit, as already discussed above; Figure 2 shows a capacitive charge pump circuit made with bipolar components, according to a known technique, as already discussed above; Figure 3 shows a capacitive charge pump circuit made with field effect transistors (CMOS) as already described above; Figure 4 shows a capacitive charge pump circuit made according to the present invention by employing bipolar and field effect components. With reference to Figure 4, the basic capacitive charge pump circuit may be identified in functional terms, by the pair of driving switches formed by the complementary pair of bipolar transistors T2 (npn) and T3 (pnp), by the charge transfer capacitor C1 by the diode T7, by the charge storing output capacitor C2 and by the circuit that functionally substitutes the first diode (D1 in Figures 1 and 2) and which in the depicted example is composed of the transistor T4 driven by the switching signal CK through an inverting stage formed by the transistor T6, by the limiting resistance R1 and by the diode T5. Phase opposition driving of the complementary pair T2 and T3 may be implemented, according to a common practice, through an inverter I1, the current generator I and the diode T1. Of course, in this case the inverter I1 must also be realized by a bipolar stage, because it must be capable of switching also at exceptionally low supply voltage. Under conditions of low supply voltage V S and in presence of a low output voltage V OUT , for example when switching-on the circuit or following an abnormal heavy absorption by the circuitry powered at the boosted output voltage V OUT produced by the circuit, the output voltage V OUT may be lower than the minimum voltage sufficient to bring to a state of conduction the field effect transistor or transistors which compose the stage I2, which is powered with the output voltage V OUT or, in case also the stage I2 is realized with bipolar transistors, the MOS pair M1 and M2. In these conditions, the MOS transistors M1 and M2 remain in an OFF state. When the driving signal CK which controls the operation of the charge pump circuit is high, T3, T6 and T4 are ON, therefore the capacitance C1 charges to a voltage given by: V C1 = V S - V CEsat(T3) - V CEsat(T4) . Basically, the transistor T4 substitutes the function of the diode D1 of the circuits of Figures 1 and 2. When the driving signal CK goes low, the node A assumes a voltage given by: V A = V S - V CEsat(T2) while the node B, which is not charged by any current in view of the fact that T6 is off, reaches the voltage given by: V B = V S - V CEsat(T2) + V S - V CEsat(T3) - V CEsat(T4) = 2V S - 2V CEsat(pnpT2 and T4) - V CEsat(npnT3) . Therefore, through the diode T7, the output capacitance C2 will tend to charge up to a voltage that in a steady state (open circuit) condition is given by: V OUT = V B - V diodeT7 = 2V S - 2V CEsat(pnp) - V CEsat(npn) - V diode . Viceversa, in conditions of operation with a low supply voltage but in presence of an output voltage V OUT sufficient to permit the turning-on of the field effect transistor or transistors, the CMOS pair of transistors M1 and M2 will be driven in phase with the driving signal CK. The turning-on of the MOS transistor M1 in phase with the transistor T3, forces the node A to ground potential, practically nullifying the voltage drop (V CEsat(T3) ) on the bipolar transistor T3, while the turning-on of the MOS transistor M2 in phase with the charge transfer from the capacitor C1 to the capacitor C2, substantially nullifies the voltage drop on the diode T3. In these precarious supply voltage conditions, through a combined action of the bipolar and CMOS devices, it is possible to obtain an output voltage that, in a steady state and open circuit conditions, reaches the value given by: V OUT = 2V S - 2V CEsat(pnp) . By considering that the voltage V CEsat of lateral pnp transistors is notably low, that is below about 100mV, the circuit is capable of achieving a substantial doubling of the supply voltage also when the latter is exceptionally low, and in practice would not allow a CMOS capacitive charge pump circuit of the known type, as depicted in Figure 3, to function. By assuming a practical embodiment characterized by the following parameters: V CEsat(pnp) = 50mV V CEsat(npn) = 200mV V BE = 0.7V and a supply voltage V S = 1.2V, the circuit of the invention, that is the bipolar part thereof, permits to reach an output voltage given by: V OUT = 2.4 - o.3 - 0.7 = 1.4V Upon activation, with the rise of the output voltage V OUT toward 1.4V, of the CMOS part of the circuit, the output voltage reaches a steady state (open circuit) value given by: V OUT = 2.4 -0.1 = 2.3V In practice, the circuit permits to obtain an output voltage that is substantially double the supply voltage also when the latter is too low for ensuring functioning of a CMOS circuit. The capacitive charge pump circuit of the invention may be employed as a voltage duplicator and also as the first stage or input stage of a voltage multiplier composed by a plurality of voltage duplicating stages in cascade.

Claims: A capacitive charge pump circuit for low supply voltage functionally composed of a charge transfer capacitor (C1) connected in series with a first bipolar transistor (T2) and a diode (T7) between a supply node (V S ) and an output node (V OUT) , to which a storage output capacitor (C2) is connected, and a second bipolar transistors (T3), said first and second bipolar transistor being switches, driven in phase opposition, of said charge transfer capacitor, characterized by comprising a first field effect transistor (M1) connected in parallel and driven in phase with the bipolar transistor (T3) of said pair which conducts during a charge phase of said charge transfer capacitance (C1), functionally capable of nullifying the saturation voltage of said bipolar transistor (T3). a second field effect transistor (M2) connected in parallel with the charge diode (T7) of said storage output capacitor (C2) and driven in phase with the charge transfer from said charge transfer capacitor (C1) to said storage capacitor (C2), functionally capable of nullifying the voltage drop on said diode (T7); means (T8, R2, I2) responsive to the rise of the voltage of said output node (V OUT ) above a minimum boosted level, capable of driving said field effect transistors (M1, M2) and to allow a substantial duplication of the supply voltage (V S ). A charge pump circuit as defined in claim 1, wherein said means are constituted by a first, level-shifting stage composed of a bipolar transistor (T8-R2) driven by a clock signal of the charge pump circuit and by at least a second inverting stage (I2) powered at the voltage (V OUT ) present on said output node. A charge pump circuit as defined in claim 2, wherein said second inverting stage is a CMOS inverter. A charge pump circuit as defined in claim 1, wherein a bipolar transistor (T4) having a first current terminal connected to the supply node (V S ), a second current terminal connected to said intermediate node (B) and a base connected to a node of a driving circuit comprising a forward biased diode (T5) connected between the supply node (V S ) and the base of the transistor, a limiting resistance (R1) and a fourth bipolar transistor (T6) driven by said clock signal, functionally connected between said limiting resistance (R1) and a common ground node of the circuit. A charge pump circuit as defined in claim 1, wherein the transistors of said pair of bipolar transistors (T2, T3) are complementary, lateral transistors. A voltage multiplier composed of a plurality of capacitive charge pump circuits in cascade, characterized by the fact that at least the charge pump circuit constituting the first or input stage of said plurality of circuits, is a circuit as defined in anyone of the preceding claims.

Inventors: ROSSI DOMENICO, TAHARA HISASHI, ROSSI, DOMENICO, TAHARA, HISASHI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H02M3
IPC codes: H01L27, H01L21, H01L29, H02M3